Chronic Intrinsic Transient Tracheal Occlusion Elicits Diaphragmatic Muscle Fiber Remodeling in Conscious Rodents

Abstract

BackgroundAlthough the prevalence of inspiratory muscle strength training has increased in clinical medicine, its effect on diaphragm fiber remodeling is not well-understood and no relevant animal respiratory muscle strength training-rehabilitation experimental models exist. We tested the postulate that intrinsic transient tracheal occlusion (ITTO) conditioning in conscious animals would provide a novel experimental model of respiratory muscle strength training, and used significant increases in diaphragmatic fiber cross-sectional area (CSA) as the primary outcome measure. We hypothesized that ITTO would increase costal diaphragm fiber CSA and further hypothesized a greater duration and magnitude of occlusions would amplify remodeling.Methodology/Principal FindingsSprague-Dawley rats underwent surgical placement of a tracheal cuff and were randomly assigned to receive daily either 10-minute sessions of ITTO, extended-duration, 20-minute ITTO (ITTO-20), partial obstruction with 50% of cuff inflation pressure (ITTO-PAR) or observation (SHAM) over two weeks. After the interventions, fiber morphology, myosin heavy chain composition and CSA were examined in the crural and ventral, medial, and dorsal costal regions. In the medial costal diaphragm, with ITTO, type IIx/b fibers were 26% larger in the medial costal diaphragm (p<0.01) and 24% larger in the crural diaphragm (p<0.05). No significant changes in fiber composition or morphology were detected. ITTO-20 sessions also yielded significant increases in medial costal fiber cross-sectional area, but the effects were not greater than those elicited by 10-minute sessions. On the other hand, ITTO-PAR resulted in partial airway obstruction and did not generate fiber hypertrophy.Conclusions/SignificanceThe results suggest that the magnitude of the load was more influential in altering fiber cross-sectional area than extended-duration conditioning sessions. The results also indicated that ITTO was associated with type II fiber hypertrophy in the medial costal region of the diaphragm and may be an advantageous experimental model of clinical respiratory muscle strength training.

Intrinsic transient tracheal occlusion training and myogenic remodeling of rodent parasternal intercostal fibers

“…Next, we compared the presence of myogenic biomarkers in the parasternal intercostals to the diaphragm after either ITTO or sham training. These experiments were an extension of our initial work on the ITTO model of inspiratory training and its use to study diaphragmatic hypertrophy [16]. The experimental procedures were approved and overseen by the University of Florida Institutional Animal Care and Use Committee.…”

“…After a recovery period of 5-7 days, animals were assigned to an 11-session training intervention similar to our previous reports [15,16,24]. Experimental sessions occurred five days per week in the morning, and lasted approximately 20 minutes.…”

“…Our laboratory found that acute bouts of intrinsic, transient tracheal occlusion (ITTO) in anesthetized animals appeared to induce progressive load compensation responses throughout the respiratory pump, as measured by an elevated diaphragm EMG activity, increased inspiratory time, and increased esophageal pressure [14]. When repeated for several days in conscious animals, ITTO load compensation responses also promote neural plasticity [15] and diaphragmatic fast fiber hypertrophy [16]. It is not known whether the plasticity to ITTO observed in diaphragmatic muscle fibers can be similarly achieved in the parasternal intercostals.…”

“…ITTO is advantageous, because it eliminates the need for permanent tracheostomies or tracheal banding techniques [15]. We recently reported the ITTO training model and its resultant hypertrophy in diaphragmatic type IIx/b fibers [16]. Mechanical strain promotes muscle fiber hypertrophy by facilitating protein synthesis pathways [18], and it also can activate satellite cells, muscle progenitors anatomically and functionally distinct from other myonuclei [19,20].…”

Diaphragm abnormalities in heart failure and aging: mechanisms and integration of cardiovascular and respiratory pathophysiology

“…Inspiratory muscle training increases diaphragm thickness in CHF patients and old subjects [131,200], suggesting fiber hypertrophy similar to that seen in healthy young rats undergoing a protocol that simulates IMT [201–203]. This approach also heightens diaphragm citrate synthase and cytochrome c oxidase activities in animals [204,205], suggesting greater mitochondrial volume density post-training.…”